The invention described herein relates to nuclear reactor fuel rods and more particularly to a method for verifying that the production specifications for internal pressure in fuel rods is being met and maintained.
The zircaloy, stainless steel or other fuel rods used in nuclear reactors are internally pressurized to about 500 psi. with a gas, such as helium, to facilitate the transfer of heat from fuel pellets in the rod to the zircaloy tubing or cladding where it is carried away by coolant circulated across the reactor fuel rods. Also, since the coolant circulates under a pressure of about 2200 psi. the internal pressure in fuel rods is used to help offset the external water pressure tending to collapse the fuel rod tubing.
Since production processes include pressurizing the rods, for quality control reasons, it is necessary to measure the pressure in representative samples of finished fuel rods to make certain that the production specifications for internal pressure is being met and maintained. It also is equally important that the test not destroy the fuel rod because of its relatively high monetary value.
The major problem encountered in measuring fuel rod internal pressure without rod destruction, is that a high degree of accuracy must be brought to the measurement process. Typical production specifications call for charging fuel rods to about 500 psi. and then maintaining a plus or minus 15 psi. tolerance from that pressure level. However, quality control engineers traditionally will seek an accuracy in the quality control test of an order of magnitude better, or in this example, plus or minus 1.5 psi. In a fuel rod charged to 500 psi. internal pressure, this 1.5 psi. value represents a measurement accuracy of plus or minus 0.3% of value. For this reason, attempts to make accurate non-invasive measurements have not been very successful. Therefore, one alternative is to pierce the wall of a rod, make an accurate pressure measurement and reweld the rod.
Although piercing the rod and discharging the gas therein into a chamber of known volume is possible, measuring the pressure to the required accuracy is still difficult because the rod void volume is known only to an accuracy of about plus or minus 20%. Decompression of the rod gas into a known volume therefore still requires subsequent precise measurements of the rod void volume. This can be done in a variety of ways, for example, refilling the rod to a known pressure, and without sealing the rod, decompressing the rod gas into a chamber of known volume to determine rod void volume; or, by utilizing a double decompression technique wherein the combined rod-chamber volume is subsequently decompressed into a second chamber of known volume. However, these techniques still will not provide the degree of accuracy necessary for determining on a sampling basis, whether fuel rods are consistently being charged to a gas pressure which meets production requirements.